Many different acoustic resonator devices have been developed. Known Bulk Acoustic Wave (BAW) resonators include a single piezoelectric layer disposed between two electrodes. Monolithic BAW resonators commonly are referred to as Thin Film Bulk Acoustic Wave Resonators (FBARs). Stacked Crystal Filters (SCFs) devices typically include two piezoelectric layers disposed between top and bottom electrode layers and separated from each other by a middle electrode, which typically is used as a grounding electrode.
A BAW resonator has a resonant frequency that is determined primarily by the thickness of the piezoelectric layer and secondarily by the thicknesses and the materials used for other the layers. A BAW resonator typically is acoustically isolated from the supporting substrate by an isolation structure, such as a cavity formed under a membrane supporting a BAW resonator (see, e.g., U.S. Pat. No. 5,873,253) or an acoustic mirror consisting of a stack of layers alternately formed of high and low acoustic impedance materials and having respective thicknesses of approximately one-quarter of the target resonant frequency of the device (see, e.g., K. M. Lakin, G. R. Kline, K. J. McKarron, “Development of Minature Filters for Wireless Applications,” IEEE Microwave Theory & Techniques Symposium Digest, pp. 883-886, (1995)).
BAW resonators often are used as components of a radiofrequency (RF) filter that may be used in, for example, mobile telephones. A common RF ladder filter, for example, includes a series BAW resonator connected in series with the signal to be filtered and a shunt BAW resonator connected in parallel to shunt the signal to be filtered. Some RF ladder filters include a series combination of multiple pairs of series and shunt BAW resonators. The resonant frequencies of the resonators of a series and shunt resonator pair are shifted slightly with respect to each other to form a passband characterized by a center frequency (i.e., a frequency midway between passband edges corresponding to the series resonance frequency of the series resonator and the parallel resonant frequency of the shunt resonator) (see, e.g., R. C. Ruby, P. Bradley, Y. Oshmyansky, A. Chien, J. D. Larson III, “Thin Film Bulk Acoustic Resonators (FBAR) for Wireless Applications,” 2001 IEEE Ultrasonics Symposium, Atlanta, Ga., Oct. 8-10, 2001, paper 3E-3, pp. 813-821).